an overview of microwave design considerations for

an overview of microwave design considerations for
AN OVERVIEW OF
MICROWAVE DESIGN
CONSIDERATIONS FOR
SWEPT SOURCES
ARLEN DETHLEFSEN
NETWORK MEASUREMENTS DIVISION
1400 FOUNTAIN GROVE PARKWAY
SANTA ROSA, CALIFORNIA 95401
Rf ~ Microwave
Measurement
Symposium
and
Exhibition
Flin-
HEWLETT
~~ PACKARD
www.HPARCHIVE.com
INTRODUCTION
C'
Microwave design and testing is highly dependent upon
the
use of microwave swept sources.
This paper
describes some of the design considerations necessary
to' achieve superior performance in a microwave Swept
Source
used
for
design
and production testing
applications.
Many of these concepts would apply in the general sense
to any electronically tuned microwave source.
AN OVERVIEW OF
MICROWAVE DESIGN
CONSIDERATIONS FOR
SWEPT SOURCES
The performance of a microwave
dependent on three major areas:
swept source is highly
1.
The block diagram concept.
2.
The microwave components used in the source.
3.
The control and drive circuitry.
This presentation will focus on the first two of these
areas.
MICROWAVE SWEPT SOURCE
DESIGN CONSIDERATIONS
1. BLOCK DIAGRAM
2. MICROWAVE COMPONENTS
3. CONTROL AND DRIVE CIRCUITRY
O'---
-----J
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There are many block diagram concepts that can be
considered for a swept source. We will look at some of
the
more
commonly used concepts and review the
advantages
of
each.
When considering the block
diagrams, the performance parameters shown here have to
be kept in mind.
I;
The designer has to make decisions on the relative
importance of each of these performance perameters in
choosing the appropriate block diagram concept.
The design of the microwave components as well as the
drive an control circuitry would also have considerable
impact on these parameters.
PARAMETERS TO CONSIDER
WHEN CHOOSING A BLOCK DIAGRAM
FOR A SWEPT MICROWAVE SOURCE
Let's now look at some of the block diagram concepts
and determine how these various configurations would
effect the performance of the source.
1.
2.
3.
4.
5.
6.
7.
8.
FREQUENCY COVERAGE
OUTPUT POWER
FREQUENCY ACCURACY AND DRIFT
HARMONIC AND SPURIOUS SIGNALS
RESIDUAL FM
MODULATION REQUIREMEMENTS
RELIBILITY
COST
The block diagrams may be placed into these four basic
categories.
Category A and B cover a single band of frequencies
using a fundamental oscillator or an oscillator driving
a single harmonic multiplier. Category C & D are block
diagrams for sources covering frequency ranges which
can not be spanned by a single fundamental oscillator.
BLOCK DIAGRAM CATEGORIES
FOR MICROWAVE SWEPT SOURCES
SINGLE BAND'
MULTI/BAND'
~~
FUNDAMENTAL
OSCILLATORS
FREQUENCY
MULTIPLACATION
I
B
I~
DEFINITIONS:
• WOULD NORMALLY REQUIRE ONE FUNDAMENTAL OSCILLATOR
TO COVER THE DESIRED BAND OF fREQUENcrES.
•• WOULD NORMALLV REQUIRE TWO OR MORE FUNDAMENTAL
OSCILLATORS OR MULTIPLICATION BY MORE THAN ONE HARMONIC
NUMBER TO COVER THE DESIRED BAND OF FREOUENCIES.
2
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Shown
A.
here is a block
diagram that fits into Category
This is the most basic and has the advantage of lowest
cost and highest reliability.
The output power would
be relatively low. Harmonics would be relatively high
and FM incidental to AM would be high because the
oscillator
is not sufficiently isolated from the
amplitude modulator.
CATEGORY A
(SINGLE BAND FUNDAMENTAL OSCILLATORS)
ELECTRONICALLY
CONTROLLED
OSCILLATOR
"Ul--------l
f----------(
'----.----'
~~TPUT
DIRECTIONAL
COUPLER/DETECTOR
This is identical to the previous diagram with the
exception that an amplifier or isolator is added to
isolate the amplitude modulator from the oscillator.
This addition greatly reduces the Incidental FM.
The use of the amplifier has two potential advantages
over the use of the isolator:
CATEGORY A
(SINGLE BAND FUNDAMENTAL OSCILLATORS)
1.
Output power would be increased.
2.
Harmonics from the oscillator could be improved if
the amplifier were designed to have a negative gain
slope as a function of frequency.
ISOLATOR
ELECTRONICALLY
CONTROLLED
OSCILLATOR
rtl
~
RF
OUTPUT
DIRECTIONAL
COUPLERI
DETECTOR
3
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This diagram has another amplifier added after the
amplitude modulator for highest output power.
CATEGORY A
(SINGLE BAND FUNDAMENTAL OSCILLATORS)
ELECTRONICALLY
CONTROLLED
OSCILLATOR
>-----< ~~TPUT
Here a filter has been added to reduce the harmonic
output signals. This filter can be a low pass or band
pass filter if the band of frequencies to be covered is
less than an octave. If the band of frequencies is
greater than an octave. the filter could then be a YIG
tuned band pass filter which is controlled by circuitry
to track .the oscillator.
More output power can be
obtained by placing an amplifier with a filter after
the modulator.
I f this is done.
the filter ahead of
the modulator could be eliminated.
CATEGORY A
(SINGLE BAND FUNDAMENTAL OSCILLATORS)
AMP
ELECTRONICALLY
CONTROLLED
OSCILLATOR
,----,-,..,....--,M
(
) >--< ~~TPUT
DIRECTIONAL
COUPLER/DETECTOR
-lOWPASS OR BANDPASS IF BAND IS lESS THAN AN OCTAVE.
ELECTRONICALLY TUNED FILTER IF BAND IS MORE THAN AN OCTAVE.
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Let's now look at two category B block diagrams.
This category is particularly useful when the frequency
of operation is high enough to make the multiplication
approach more desirable from a performance/cost point
of view.
It also allows for a convenient way to
provide an auxiliary output at a sub multiple of the
output
frequency.
This
output
is
useful for
phase-locking the source or for using a frequency
counter.
BLOCK DIAGRAM CATEGORIES
FOR MICROWAVE SWEPT SOURCES
SINGLE BAND'
MULTI/BANIi'
~~
FUNDAMENTAL
OSCILLATORS
FREQUENCY
MULTlPLACATlON
I :.. ]':::/j~
······:-:-I~
DEFINITIONS:
• WOULD HORMALlV REQUIRE ONE FUNDAMENTAL OSCILLATOR
TO COVER THE DESIRED BAND OF FREQUENCIES.
... WOULD NQRMALlV REQUIRE TWO OR MORE fUNDAMENTAL
OSCILLATORS OR MULTIPLICATION BY MORE THAN ONE HARMONIC
NUMBER TO COVER THE DESIRED BAND OF FREQUENCIES.
This Category B diagram has the modulator and amplifier
ahead of the multiplication process. The filter may be
a fixed broadband band-pass filter provided f2 is less
than f1(N+1)/N. If f2 is greater than f1(N+1)/N, the
filter would need to be a tunable bandpass filter.
f1
is defined as the lowest output frequency, f2 is the
highest output frequency and N is the multiplication
number.
CATEGORY B
(SINGLE BAND FREQUENCY MULTIPlER)
It is also possible to design multipliers to balance
out the odd or even harmonics. This minimizes or
eliminates the need for a bandpass filter.
ELECTRONICALLY
CONTROLLED
OSCILLATOR
RF
OUTPUT
.. FixeD BANDPASS FIL.TER IF 12
< 1,
IN-~1)
TUNABLE BANDPASS FILTER IF '2> f1
(~I
f,-LOWEST Dupur fREQUENCY,
f 2 ", HIGHEST OUTPUT FREQUENCY.
N .. MUL.TlPLICATlON NUMBER.
5
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In this diagram, you will notice that the modulator and
amplifier are after the multiplication process. The
previous diagram had the advantage of modulating and
amplifying at lower microwave frequencies.
This configuration has the potential for higher output
power and, in the case of some types of multipliers.
the unwanted harmonics can be more easily controlled as
the output power is varied.
CATEGORY B
(SINGLE BAND FREQUENCY MULTIPIER)
n
ELECTRONICALLY
CONTROUED
OSCILLATOR
~ >--< ~~TPUT
DIRECTIONAL
COUPLER/DETECTOR
Let's
area.
now look at the block diagrams in the multi-band
BLOCK DIAGRAM CATEGORIES
FOR MICROWAVE SWEPT SOURCES
SINGLE BAND'
MULTI/BANC'
FUNDAMENTAL
OSCILLATORS
FREQUENCY
MULTIPLACATION
DEFINITIONS:
• WOULD NORMALL.Y REQUIRE ONE FUNDAMENTAL OSCILLATOR
TO COVER THE DESIRED BAND OF FREQUENCIES.
.. WOULD NORMALLY REOUIRE TWO OR MORE FUNOAMENTAl
OSCILLATORS OR MULTIPLICATION BY MORE THAN ONE HARMONIC
NUMBER TO COVER THE DESIRED BAND OF FREQUENCIES.
6
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This block diagram using fundamental oscillators has
the advantage that the harmonics can be more easily
filtered and there are no harmonic products below the
desired signal.
The oscillators and amplifiers are
designed to cover discrete frequency bands but the
modulator and switch must operate over the lowest to
the highest frequency of interest.
CATEGORY C
(MULTI-BAND FUNDAMENTAL OSCILLATORS)
ELECTRONICALLY
CONTROLLEO
OSCILLATOR
RF
OUTPUT
DIRECTIONAL
COUPLER!
DETECTOR
The frequency multiplier approach has the advantage of
better frequency accuracy and less frequency drift due
to
temperature.
This is because the oscillator
operates at a lower microwave frequency where it is
easier to design a stable, linear oscillator with low
hysteresis.
This will become more apparent when we
review the component designs.
The relatively low
frequency of the fundamental oscillator can easily be
coupled to an auxilIary port for frequency measurements
or for phase locking to a stable reference. The only
components
which
require designs at the highest
microwave frequencies are the multiplier/filter and the
directional coupler. This concept also minimizes the
number of microwave components and drive circuitry.
CATEGORY D
(MULTI-BAND FREQUENCY MULTIPIER)
Now let's focus on the various microwave components
used in these block diagrams and how their design
effects the overall performance of the product.
RF
OUTPUT
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Shown here are the six major microwave components that
are used in swept sources.
Most of the effort will be spent on the selection and
the
microwave
oscillator
since the
design
of
oscillators performance has a significant bearing on
most of the electrical parameters of the swept source.
MAJOR MICROWAVE COMPONENTS
USED IN SWEPT SOURCES
- OSCILLATOR
- AMPLIFIER
AM MODULATOR
MULTIPLIERS
FILTERS
DIRECTIONAL
COUPLER/DETECTOR
Here is a list of important parameters for electrically
tuned microwave Oscillators. The oscillator design can
be broken down into four areas:
the tuning device.
active devices, circuit design and mechanical design.
As you can see. the decisions made for each of these
areas impact most if not all of the performance
parameters.
Let's take a brief look at each of these
areas.
DESIGN DECISIONS THAT EFFECT ELECTRICALLY TUNED
MICROWAVE OSCILLATOR PERFORMANCE PARAMETERS
Devices and Designs that effect the paraneter
Paokage and
Parcrneter
Tuning
Device
Active
Devices
Circuit
Design
Mechanical
Design
Operating frequency
X
X
X
X
Tuning Range
X
X
X
X
Output Power
X
X
X
X
Tuning Signal Linearity
X
X
X
X
frequency Accuracy
X
X
X
X
Frequency Changes
X
X
X
X
X
X
YS.
temperature
Tuning Sensitivity
X
Harmonic and
X
X
X
Noise and Residual FM
X
X
X
Magnetic Susceptibility
X
X
X
Pulling/Pushing
X
X
X
Weight & Size
X
X
X
X
Power ConslIJlption
X
X
X
14
11
14
Spurious outputs
Total Paraneters
Effected
8
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10
The
tuning
device chosen for an oscillator has
considerable impact on the oscillators performance
since it effects virtually every parameter.
COMPARISION OF THE YIG SPHERE AND VARACTOR DIODE
FOR TUNING MICROWAVE OSCILLATORS
Dev; ce
Performance Parameter
We have indicated where a specific device
inherent advantage for a given parameter.
VARACTOR
YIG
Operating Frequency
"'I - 40 GHz'
<20 GHz
Tuning Range
Multi-octave·
Octave
Tuning Rate
Slow
Fast*
Tuning Linearity
Linear*
Exponential
.
.
Frequency Accuracy
Noise & Residual FM
Power Consumption
Magnet; c
Tuning Method
has an
In summary, the YIG tuning device is most suitable to
applications requiring high frequencies, broad tuning
ranges, good nOlse performance and linear change in
output frequency as a function of the tuning signal.
The
varactor tuning device is most suitable for
applications requiring fast tuning or where there is a
size, cost, or power consumption constraint.
The power consumption and size of YIG tuned oscillators
generally do not present a problem for most swept
sources.
Sweep speeds on the order of 10 to 30
milliseconds
are
generally
accpetable
for most
appl ica tions.
The refore, the YIG tuned oscilla tor,
with its advantages in frequency of operation, tuning
range, noise and tuning linearity, has become the
oscillator
that
is
predominantly used in swept
microwave sources.
.
.
Weight & Size
The most commonly used electronic tuning devices for
microwave oscillators is the varactor diode and the
Yttrium-Iron-Garnet (YIG) sphere.
Shown here is a
comparison of these two devices.
The
following
discussions
on
oscillator design
considerations will therefore be limited to the YIG
tuned oscillator.
Voltage
Field
* Device has an inherent advantage on this parameter.
The
active
devices used in broadband YIG tuned
oscillators are shown in this diagram. Below 10 GHz,
either the Bipolar or FET devices are generally used.
Bipolar transistors presently have an advantage in the
area
of close-in phase noise. so for low noise
applications, the Bipolar devices are generally used.
ACTIVE DEVICES USED IN WIDEBAND YIG TUNED OSCILLATORS
In the past, bulk GaAs diodes have been predominantly
used at frequencies above 8 GHz.
However, with the
advent of 26 GHz FET devices, many new applications
above 8 GHz will be using the FET transistors because
the circuit may be designed to tune over greater than
octave frequency ranges. It also has the advantage of
reqUiring less supply power.
Typi cal Osci nator
Devi ce
Performance Characteristics
Bi-Polar
Useab1e to 10 GHz
Greater than Octave operating range can be achieved
lowest close-in Phase Noise
Good eff; ci ency
Output Power 10 ""
Trans; star
Field-Effect
Transistor
Useab 1e to 26 GHz
Greater than Octave operating range can be achieved
Good efficiency
low Phase Noise
Output Power
Bulk
GaAs diode
lOnw
Useable B to 40 GHz
Poor eff; c;ency
Power output approximately 10 to 40
11\'1
low Phase No; se
Octave Tun;"9 Range
9
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Broadband
microwave
oscillator
designs
using
transistors
are
normally
designed using circuit
configurations shown here.
This design allows for
maximum bandwidth while still achieving reasonable
performance in the areas of output power. noise. and
harmonic level.
Oscillators using this topology are
presently available that span 1.5 to 2 octaves. With
improvements in devices and by using multiple tuning
elements.
further
increases in bandwidth can be
expected in the future.
Listed below are some reference articles that deal with
wideband microwave oscillator design.
MICROWAVE BROAD BAND YIG TUNED
TRANSISTOR OSCILLATOR CIRCUIT TOPOLOGIES
Oscillator Design References:
BIPOLAR
OSCILLATOR TOPOLOGY
1. Ganesh R. Basawapatna and Roger B. Stancliff, "A
Unified Approach to the Design of Wide-band Microwave
Solid-state Oscillators" IEEE Trans. Microwave Theory
Tech. Vol Mtt-21. No.5. pp 319 - 385, May 1979.
MESFET
OSCILLATOR TOPOLOGY
2. James C. Papp and Yoshiomi 1. Koyano. "An 8 - 18
GHz YIG-Tuned FET Oscillator" IEEE Trans. Microwave
Theory Tech. Vol MTT-28, No.7. pp 762.
The remaining area of the oscillator design that has to
be addressed is the magnetic structure. This is a key
element in the design since it affects such things as
tuning linearity. frequency drift with temperature and
tuning sensitivity.
The basic structure required to provide
field for the YIG Sphere is shown here.
It consists of a magne tic
sphere, a driver coil and
oscillator circuit.
core. a gap
a means to
a magnetic
for the YIG
support the
Let's take a brief look at some of the key properties
of
electromagnets
and
see how they effect the
performance of the YIG tuned oscillator.
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BASIC MAGNETIC STRUCTURE
FOR TUNING YIG OSCILLATORS
MAGNETIC CORE \
o
These are the four primary parameters of magnetic
structures that affect oscillator performance.
PARAMETERS
OF MAGNETIC STRUCTURES
THAT EFFECT
OSCILLATOR PERFORMANCE
1.
2.
3.
4.
SWEEP DELAY
HYSTERESIS
LINEARITY & SATURATION
TUNING SENSITIVITY
Sweep delay is defined as the frequency lag relative to
the tuning current under continuous sweep conditions.
"Delay",
in
this
context,
represents frequency
inaccuracy as a function of tuning speed. As shown
here,
this delay increases with increased tuning
speeds. Typical numbers for uncorrected delay would be
100 MHz for an oscillator in the 8 GHz range sweeping
at a 10 ms sweep rate.
Choosing a magnetic material
with high resisitivity minimizes this effect. However,
in order to maintain good frequency accuracy as a
function of sweep speeds, additional corrections are
normally required in th oscillator drive circuitry.
SWEEP DELAY OF YIG TUNED OSCILLATORS
>u
zw
::;)
ow
a:
u.
TUNING CURRENT
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Hysteresis
is defined as the maximum differential
(at
a fixed-coil current) due to the
hysteresis of the magnetic circuit when tuned in both
directions through the operating range. Hysteresis can
be
minimized
by carefully choosing the magnetic
material.
As shown, hysteresis increases with wider
operating ranges. It also increases with increases in
flux density and therefore higher frequency YIG tuned
oscillators
have
larger
values
of
hysteresis.
Hysteresis has a direct bearing on the frequency
accuracy of the tuned oscillator since there is no
simple way of compensating for this phenomenon with
external circuitry.
freq~ency
HYSTERESIS OF YIG TUNED OSCILLATORS
>-
U
Z
HYSTERESIS
T
NARROW
OPERATING
RANGE
W
~
aw
a:
II.
t
1
COIL CURRENT
Saturation occurs when increases in coil current do not
produce further linear increases in the flux density.
The saturation level depends on the properties of the
magnetic material as well as the design of the magnetic
structure.
Unfortunately, magnetic material which is
chosen for high saturation levels has properties which
increase the hysteresis of the magnet.
The saturation level determines the maximum frequency
to which the oscillator may be tuned and also has a
bearing on oscillator linearity since any deviations
from a straight line relationship between flux density
and coil current will effect frequency accuracy as a
function of the tuning signal.
Frequency linearity is also affected by
device. circuit design and active devices.
MAGNETIC SATURATION AND LINEARITY
the tuning
(
Careful circuit and magnetic designs are essential in
this area to produce good performance.
>-
u
SATURATION LEVEL
----
-
zw
~
aw
a:
II.
COIL CURRENT
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Tuning sensitivity is defined as the differential
current required to tune across the operating frequency
range divided by the frequency range. The sensitivity
is a function of the number of turns and the width of
the gap.
In order to minimize the power necessary to tune the
oscillator. it is essential that the gap be kept as
small as possible. The mechanical design of the magnet
must also be such that the gap size does not vary as a
function
of
temperature
since this would cause
inaccuracies in the frequency of the source.
TUNING SENSITIVITY 0<. NIS
S
N = # OF TURNS ON TUNING COIL
OF THE GAP IN THE MAGNETIC STRUCTURE
= WIDTH
•
COIL
The two magnetic structures that are normally used are
shown here.
The single ended design is simpler and
therefore less costly. The double ended design has the
advantage of better hysteresis and is capable of higher
saturation levels since there are fewer leakage paths
for the flux.
The double ended design is also less
susceptible to externally applied magnetic fields.
CROSS SECTION OF BASIC MAGNETIC STRUCTURES
USED FOR YIG TUNED MICROWAVE OSCILLATORS
......
r
MAGNETIC MATERIAL
--t._-,
"'
-
-
YIG
SPHERE~~=---J
CIRCUIT -
"
DRIVER
COIL
YIG
SPHERE
- ....
-
SINGLE-ENDED DESIGN
~
~
DOUBLE-ENDED DESIGN
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An example of a 2 to
oscillator is shown here.
8.4
GHz
Bipolar transistor
The magnets are made of a low hysteresis material. The
coils are layer-wound which minimizes the size of the
magnet.
This structure has a saturation frequency in
excess of 12 GHz.
}
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The oscillator transistor is a Silicon Bipolar device
followed with a FET buffer amplifier.
It uses a 660
micron diameter sphere (26 mil) with an unloaded Q of
1700.
The sphere is mounted on a sapphire rod and oriented on
a temperature compensated axis. In addition. it is
kept at a constant temperature with a thermostatically
controlled heater. This keeps the post tuning drift of
the oscillator under 100 KHz.
The sphere and devices were specially
Hewlett Packard for this product.
designed at
The oscillator actually operates between 1.8 and 8.6
GHz and its basic performance is listed here.
TYPICAL PERFORMANCE OF 2-8.4 GHz
TRANSISTOR YIG TUNED OSCILLATOR
OUTPUT POWER
HARMONICS
TUNING SENSITIVITY
15 mW
20 dBc
24 ma/GHz
HYSTERESIS
LINEARITY
2 MHz
16 MHz
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The phase noise characteristics are
single side band noise is typically
carrier at a 10 KHz offset.
shown here. The
100 dB below the
2 - 8.4 GHz OSCILLATOR PHASE NOISE AS A FUNCTION OF FREQUENCY
FREQUENCY OFFSET FI()M CARRIER· Hz
Now let's briefly review some of the
considerations for the other components.
The key parameters for amplifiers
sources are shown here.
key
design
used in microwave
The performance requirements would vary depending on
the requirements of the specific product. However. it
is normally beneficial to achieve as broad a band of
operation as the devices and circuit design will allow.
IMPORTANT AMPLIFIER PARAMETERS
FOR USE IN SWEPT SOURCES AND
TYPICAL PERFORMANCE REQUIREMENTS
PARAMETER
FREQUENCY RANGE
OUTPUT POWER
HARMONICS
INPUT & OUTPUT MATCH
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TYPICAL
PERFORMANCE REQUIREMENTS
2:1
40
20
2:1
to 10:1
to 400 mW
to 40 dBc
V.S.W.R.
Broadband high power designs are normally best achieved
by using a design approach as shown here.
The interstage matching networks are designed such that
they provide maximum gain at the highest frequency of
operation and reduce the gain at the lower frequency to
achieve an amplifier gain that is relatively flat with
frequency.
TYPICAL BLOCK DIAGRAM
FOR A
BROADBAND MICROWAVE POWER AMPLIFIER
In
order
to
achieve sufficient power over the
broad range of frequencies. it is necessary to combine
the outputs of two or more devices. This is normally
achieved by using hybrids as shown here.
POWER COMBINING USING QUADRATURE HYBRIDS
INPUT
OUTPUT
17
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An example of a 2 to 7 GHz 0.5 watt MESFET amplifier
for use in swept sources is shown here. It has a gain
of 18 dB @ 0.5 watt output with harmonics typically 20
dB below the fundamental.
To achieve this performance. two specially designed
FET's were utilized. The 1 micron x 500 micron device
shown here was designed to have a high fmax which
simplifies broadband amplifier designs.
For references purposes. a human hair is approximately
100 microns in diameter.
1 MICRON
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X
500 MICRON X-BAND FET
This 1.5 x 1500 micron device was designed to achieve
high output power with low distortion. It can deliver
300 mw @ 6 GHz.
1.5 MICRON x 1500 MICRON LINEAR FET
The 500 micron device is used to drive the 1500 micron
device as shown here.
Two 1500 devices re combined with a quadrature hybrid
to achieve the 0.5 watt output.
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Frequency multilpliers can be categorized as shown in
this slide. Passive multiplers have no gain mechanism
while an active multiplier has the ability to provide
more output RF power at the multiplied frequency than
is provided at the input of the multiplier. Passive
multiplication is normally achieved by using rectifier
type
diodes
or
step
recovery
diodes.
Active
multiplication
is
achieved
using
field
effect
transistors.
FREQUENCY MULTIPLIER CATEGORIES
1. PASSIVE
(a) RECTIFIER DIODE
(b) STEP RECOVERY DIODE
2. ACTIVE
(a) FET
Typical
passive multipliers are shown here.
The
passive doubler is essentially a full wave rectifier
which is rich in even order harmonics. The passive
tripler is a diode limiter which is rich in odd order
harmonics.
The comb multipler using a step recovery diode has an
output wave shape that is essentially an impulse and
therefore generates a comb of frequencies of both odd
and even order.
PASSIVE MULTIPLIERS
DOUBLER
TRIPLER
~~
fo
IVV\
to
fVVVV\
~
2'0
RECTIFIER TYPE DIODES
JUlJl
310
LPF
~
COMB
IVV\
TrTT
to
.to
STEP
RECOVERY
DIODE
20
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The active doubler is a FET device which is biased to
rectify the input signal. Since the device has gain,
the output signal can be larger in magnitude than the
input signal.
BALANCED ACTIVE DOUBLER
DUAL
GATE
FET'S
RF IN
10
RF OUT
210
/
An example of a single band active doubler is shown in
this slide. The input power of the doubler is +13 dBm
as is its output.
This design has the modulator following the multiplier
and also utilizes an 18 to 26.5 GHz post amplifier.
The amplification compensates for all circuit losses.
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The devices used in this doubler are two dual gate
FET's and a 0.5 x 350 micron gate device is used in the
amplifier.
The pattern of the dual gate FET is shown
here.
1 MICRON
X
400 MICRON DUAL GATE FET
The 0.5 x 350 micron FET used for the amplifier has an
fmax of 60 GHz and is capable of delivering 40 mw @ 26
GHz.
These two devices are also special HP designs.
0.5 MICRON
22
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X
350 MICRON K BAND FET
An example of a frequency multiplier using a
recovery diode is shown here. The input frequency
to 7 GHz and the YIG filtered output frequency is
26.5 GHz using multiplication numbers of 2, 3, and
step
is 2
2 to
4.
The magnetic structure was designed using two different
magnetic materials.
The center body and pole tips are made of a low
saturation material while the end pieces are made of a
low hysteresis material.
The shape of the pole and
package was optimized to minimize flux leakage paths.
Thermal shorts were designed to carry heat away from
the pole tips. The magnet saturates at frequencies in
excess of 30 GHz.
23
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The 680 micron (27 mil) YIG sphere is mounted in a 254
micron (10 mil) thick sapphire substrate.
The YIG
sphere
is
kept at a constant temperature by a
thermostatically controlled heater.
The HP designed step recovery
time less than 30 ps.
diode has a transition
Typical conversion losses on the order of 10 dB are
achieved to 20 GHz and 15 dB at 26 GHz. Fractional and
subharmonics are typically 35 dB below the desired
signal and harmonics are typically 50 dB below the
desired desired signal.
This particular multiplier also has provisions for a
multiplexed 10 MHz
2.4 GHz signal so that the
assembly can deliver a 10 MHz to 26.5 GHz swept signal
from a single port.
The functions
here.
of
the
Amplitude Modulator
are shown
Items
and 2 are virtually essential for all modern
swept sources. Items 3 and 4 are normally designed to
meet the performance objectives of the source.
FUNCTIONS PROVIDED BY AM MODULATOR
1. RF LOSS CONTROL MECHANISM FOR AUTOMATIC
LEVEL CONTROL AS A FUNCTION OF FREQUENCY.
2. SETS THE LEVEL OF RF OUTPUT POWER.
3. BLANKS RF OUTPUT ON RETRACE OF
SWEEP OSCILLATOR.
4. PROVIDES MEANS OF AMPLITUDE MODULATING
THE RF SIGNAL.
(a) SINUSOIDAL AND SQUARE WAVE
(b)
24
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PULSE
Most microwave modulators today utilize the PIN diode
in
either a series or shunt configuration or a
combination
of
the
two to provide the desired
performance.
The
series and shunt versions are
completely reflective while the combination circuit can
be designed to have reasonable input and output match
specifications.
MICROWAVE AMPLITUDE MODULATOR TOPOLOGIES
SHUNT
SERIES
nIT"'"'
PIN DIODE
RFIN~RFOUT
t J
PIN DIODE
MOD BIAS
MOD BIAS
COMBINATION SERIES/SHUNT
RF IN )
}
.,
BIAS MOD
n
PIN DIODES
<RF OUT
14
1
BIAS MOD
In order to mlnlmize problems associated with modulator
input and output match changes as a function of
frequency and RF output level, it is good design to
include an input and output amplifier or isolator as
shown here.
AM MODULATOR WITH INPUT/OUTPUT BUFFERS
MOD BIAS
MOD BIAS
RF
INPUT
RF
INPUT
AMPLIFIER VERSION
ISOLATOR VERSION
25
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The
directional
functions:
1.
2.
to
coupler/detector
has
two
primary
provide a DC output that is proportional to the
RF output power.
to improve the source output match.
The output is amplified and fed back to the amplitude
modulator to achieve leveled output power as a function
of frequency.
FUNCTIONS PROVIDED
BY DIRECTIONAL COUPLER AND DETECTOR
1. PROVIDE A DC OUTPUT SIGNAL
THAT IS PROPORTIONAL TO THE
RF OUTPUT POWER.
2. IMPROVE OUTPUT SOURCE MATCH.
To achieve good levelling, the combination of coupling
loss and detector response together need to provide a
DC output that does not vary as a function of frequency
for a given output power level.
Good source match is achieved when the output connector
has a good VSWR and the coupler has high directivity.
LEVELLING AND SOU~CE MATCH DEGRADATION
CAUSED BY OUTPUT COUPLER PARAMETERS
FOWARD
SIGNAL
)COUPLER
INPUT
><
DIODE
L-
j<
COUPLER
OUTPUT
DETECTOR
OUTPUT
--'
ERROR DUE
TO COUPLER
DIRECTIVITY
26
www.HPARCHIVE.com
C
\
'
=~;:~TED
" ' - ERROR DUE
TO COUPLER
OUTPUT MATCH
In concluding, we will identify, by block diagram
category type, some current Hewlett-Packard designs of
swept
sources
to
determine what performance is
achievable using the concepts presented in this paper
and state-of-the-art microwave devices and designs.
TYP leAL PERFORMANCE
OF THE
HP 86260A SWEPT SOURCE
02.4-18.0 Gft:r.)
!LOCl DIACRAM CATEGORY
osc.
TUNING DEVICE
YIO
OSC.
ACTIVE DEVICE
BULK
GaAS DIODE
OSC.
MAGNETIC STRUCT.
DOUHE ENDED
TYPE OF HARMONIC FILTERING
NONE
AUX. OUTPUT FOR
COUNTEI OR PHASE-LOC¥.
NO
OUTPUT POWER (.,,)
12
FREQUENCY ACCURACY
(KHz)
30
HYSTERESIS (MHz)
10
(I.H:r. PEAl
RES IDUAL FM
IN 10 I.H:r. BANDWIDTH)
I'
HARMONICS (dB BELOW
FUNDAMENTAL)
30
INTERNAL LEVELED
POWER VARIATION (dB)
For Category A, the HP 86260A is a single band swept
source using a bulk GaAs diode. Output power is 12 mw.
Frequency accuracy is 30 MHz with a hysteresis of 10
MHz.
A
+ -0.5
Another Category A unit, the HP 83545A, is a single
band unit designed for high output power. It uses a
FET transistor oscillator and typically provides 60 mw
leveled output between 5.9 and 12.4 GHz.
TYPICAL PEItFORHANCE
OF THE
HP 83545A SWEPT sou aCE
(5.9 TO 12.4 GH:r.)
SLOCl DIAGRAM CATEGORY
A
OSC.
TUNING DEVICE
YlO
OSC.
ACTIVE DEVICE
FET
OSC. MAGNETIC STlUCT.
SUCLE END
TYPE OF HARMONIC FILTERING
FIXED
LOW-PASS
AUX. OUTPUT FOR
COUNTER OR PHASE-LOCI.
NO
OUTPUT POWER (m,,)
60
FREQUENCY ACCURACY
(MHz)
I'
HYSTERESIS (Hh)
20
(1Hz PEAl.
RES IDUAL FM
IN 101Hz BANDWIDTH)
10
HARMONICS (dB BELOW
FUNDAMENTAL)
INTERNAL LEVELED
POWER VARIATION (dB)
>40
(7 -1 2GHz)
+-0.4
27
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Still another example of Category A is the HP 83540B, a
product designed for high power, good harmonics and
frequency
accuracy.
It
utilized a double-ended
oscillator structure with a tracking YIG filter to
achieve
very
low
output
harmonics
over
its
double-octave frequency range.
tYP leAL PERFORMANCE
OF THE
HP 835408 SWEPT SOURCE
(2 TO 8.4 CRt)
BLOCK DIAGIAM CATEGORY
A
osc.
TUNING DEV ICE
YlC
OSC.
ACtIVE DEY ICE
B I-POLAR
ose.
MAGNETIC STRueT.
DOUBLE
TYPE OF KARMone
AUX.
FILTERING
ENDED
YIC TUNED
NO
OUTPut FOR
COUNTEI. 01. PHASE-LOCt:;
30
OUTPut POWER (aw)
'-,
FREQUENCY ACCURACY
(KHid
1.2
HYSTERESIS (MHz)
FH
(lUz PEAl:
IH 10 KHz IIAMDW lDTH)
IES IOUAL
HARMONICS
(d. BELOW
,
'0
FUNDAMENTAL)
INTERNAL LEVELED
POWER VARIATION (dB)
For Category B, the HP 83570A is an 18 - 26.5 GHz
doubler type source with 11 mwoutput power. The
output of the fundamental oscillator, 9.0 - 13.25 GHz,
is made available as an auxiliary output. This signal
can be used as the RF sample for phase-locking or can
be applied to a microwave counter.
+-0.8
TYPICAL PERFORHANCE
OF THE
HP 83.570" SWEPT SOUICE
(18 TO 26.5 CHz)
IILU\.I: DIACUM CATEGORY
•
OSC.
TUNING DEVICE
YlC
osc.
ACTIVE DEVICE
FET
ose.
MACNETIC STlueT.
SINGLE
ENDED
FIXED
TYPE
OF HARMONIC
rlLTElING
AUX. OUtput FOR
COUNTEI OR PHASE-Lon
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YES
OUTPUT POWER (mv)
II
FREQUENCY ACCURACY
(MHd
20
HYSTERESIS (MHz)
12
(1Hz PEAl
RES IOOAL FM
IN 10 J:H:r. BANDWIDTH)
20
HARMONICS (dB BELOW
FUNDAMENTAL)
30
INTERNAL LEVELED
POwER. VAiIATIOM (d 8)
28
HICH-PASS
"'-1.2
TYPICAL
For Category D, the HP 83592B and 83595A are multi-band
sources designed to span the .01 to 20GHz and .01 to
26.5GHz bands with good power and excellent frequency
accuracy and residual FM. Output power is typically 25
mw at 20 GHz with frequency accuracy of 4 MHz. In
order to achieve this type of accuracy you will notice
that the hysteresis in all bands is typically 1.2 MHz.
This is achieved because the oscillator operates over
narrower ranges as the source is tuned to higher
frequencies.
PERFORMANCE
OF THE
HP 8359211 AND HP 835951.
SWEPT SOURCES
83595,\
8359211
BLOCK DIAGRAM CA TEeORY
f
D
D
osc.
TUNING DEVICE
VIG
VIG
osc.
ACtIVE DEVICE
81-POLAI.
Ill-POLAR
osc.
HAGNETIC
DOUIlLE ENDED
DOUBLE ENDED
TYPE OF HARHONIC FILTERING
VIG TUNED
VIG TUNED
FR.EQUENCY RANGE (GHz)
.01 TO 20
.01 TO 26.5
sTluer.
25
OUTPUT POWER. (.w)
FREQUENCY ACCURACY
(MHz)
(MHz.)
HYSTERESIS
IES IDVAL
IN 10
K"
(1Hz PEAl:
BANDW 10TH)
rM
25 TO 20 GR1
TO 26.5 GR1
,
5
1.2
1.2
3 @ 6 GR1
10 @ 20 GR1
3 @ 6 GK<
[email protected] 26. 5 GK<
lNTEINAL LEVELED
POWER VARIATION (dll)
25 BELOW 2. ,
50 ABOVE 2. ,
35
35
+-0.7
+-0.7
HARMONICALLY RELATED
(d. BELOW YU HOAKEN! At)
The aUXiliary output from this unit's fundamental
oscillator covers 2 to 6.7 GHz, yet it can be counted
or phase-locked as if it were a 26 GHz signal.
,
o.
25 BELOW 2.' GN1
50 ABOVE 2. , GR1
HARKON les (dB BELOW
FUNDAMENTAL)
The residual FM performance of this product at high
frequencies is superior to many of the single band
units because the residual FM of the 2 - 8.4 GHz
oscillator is only 3 KHz at 6 GHz.
This noise
multiplied by four yields a residual FM performance of
12 KHz at 26.5 GHz.
GM1
GR1
In summary then, we have reviewed several design
criteria necessary to achieve superior performance in
swept sources.
Most of the focus has been on the microwave block
diagrams and microwave components. However, in all
system designs it is essential that the drive and
control circuitry is carefully designed so that it does
not degrade the inherent performance of the microwave
components.
SUMMARY
As in all system designs, many compromises have to be
considered in order to have a cost effective product.
These compromises require good judgment and proper
evaluation of the important parameters. However, these
decisions should not jeopardize the reliability of the
product.
Drive & Control Circuity
In order to achieve a reliable prOduct, it is essential
that the basic building blocks have been designed and
chosen with reliability in mind and that design margins
are considered in each area of design.
Microwave Circuitry
& Interface Circuitry
- IMPORTANCE OF DRIVE AND
CONTROL CIRCUITRY
- RELIABILITY
29
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PRINTED IN U.S.A.
MAY 1982
www.HPARCHIVE.com
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